For several years now I own a rifle chambered for the old 6.5x54 K Mauser cartridge. This cartridge is not to be confused with the 6.5x54 Mannlicher – Schoenauer. The photo shows, left to right, two prewar DWM factory loads, my own handload with a 120 gr .264" bullet, and a 6.5x54 M-Sch handload.

My rifle is not a Mauser K model, but a Schmidt & Habermann, Suhl, M 21 with the original period 2 ½ x Zeiss "Zielmi" scope.

As I never found proper reloading dies, I improvised from the start. I use Sellier & Bellot 6.5x57 brass to make cases. (RWS brass is less suitable for such reforming as the case walls are thicker just below the necks). Shorten the cases to 54 mm. Resize the case bodies in an 8x57 die body, without expander of course. Set back the shoulders in a shortened 6.5x55 SE FL die until bolt barely closes. The case shoulders fireform with the first use. From then on I necksize only with the same 6.5x55 old second hand Lee die. The x55 seating die works without modification, but if you want to crimp, you have to shorten it too.
My load: 34 gr VV N140 behind a .264" 120 gr bullet for about 2450 fps, close to the original ballistics. I use .264" bullets as is, without resizing them by .003".